# Ruby Quick Reference

*Objects, blocks, iterators, regex, file I/O essentials*

> Source: Ruby Documentation (ruby-lang.org) · MIT

## Basics

### Hello World

```
puts "Hello, World!"
print "no newline"
p [1, 2, 3]   # inspect output: [1, 2, 3]
```

### Run Ruby

```
ruby script.rb          # run a file
ruby -e 'puts "hi"'    # run inline
irb                     # interactive REPL
```

### Variables

| Command | Description |
|---------|-------------|
| `name` | Local variable |
| `@name` | Instance variable |
| `@@count` | Class variable |
| `$debug` | Global variable |
| `MAX_SIZE` | Constant (uppercase by convention) |

### Types

```
42.class         # Integer
3.14.class       # Float
"hello".class    # String
true.class       # TrueClass
nil.class        # NilClass
:symbol.class    # Symbol
```

## Strings

### String Basics

```
name = "World"
puts "Hello, #{name}!"        # interpolation (double quotes)
puts 'No #{interpolation}'    # literal (single quotes)
multi = <<~HEREDOC
  indented heredoc
HEREDOC
```

### String Methods

| Command | Description |
|---------|-------------|
| `.length / .size` | Character count |
| `.upcase / .downcase` | Case conversion |
| `.strip` | Remove leading/trailing whitespace |
| `.split(',')` | Split into array |
| `.gsub(/pat/, 'rep')` | Global substitution |
| `.include?('sub')` | Check if contains substring |
| `.start_with?('pre')` | Check prefix |
| `.chars / .bytes` | Array of characters / bytes |
| `.to_i / .to_f` | Convert to integer / float |
| `.freeze` | Make string immutable |

## Arrays & Hashes

### Arrays

```
arr = [1, "two", :three]
arr << 4                  # push (append)
arr[0]                    # 1
arr[-1]                   # 4 (last element)
arr[1..2]                 # ["two", :three] (slice)
```

### Array Methods

| Command | Description |
|---------|-------------|
| `.push / .pop` | Add/remove from end |
| `.shift / .unshift` | Remove/add from beginning |
| `.flatten` | Flatten nested arrays |
| `.compact` | Remove nil values |
| `.uniq` | Remove duplicates |
| `.sort / .reverse` | Sort / reverse order |
| `.map { \|x\| x * 2 }` | Transform each element |
| `.select { \|x\| x > 0 }` | Filter elements |
| `.reduce(0) { \|sum, x\| sum + x }` | Accumulate into single value |

### Hashes

```
user = { name: "Alice", age: 30 }   # symbol keys
old = { "key" => "value" }           # string keys
user[:name]                          # "Alice"
user[:email] = "a@b.com"             # add pair
user.fetch(:name, "default")         # with default
```

### Hash Methods

| Command | Description |
|---------|-------------|
| `.keys / .values` | Array of keys / values |
| `.each { \|k, v\| }` | Iterate key-value pairs |
| `.merge(other)` | Merge two hashes |
| `.key?(k) / .value?(v)` | Check existence |
| `.select { \|k, v\| }` | Filter pairs |
| `.transform_values { \|v\| }` | Transform all values |

## Control Flow

### Conditionals

```
if score >= 90 then "A"
elsif score >= 80 then "B"
else "C"
end
puts "adult" if age >= 18     # inline if
puts "minor" unless age >= 18 # inline unless
```

### Case / When

```
case status
when :ok then puts "success"
when :error then puts "failed"
when 400..499 then puts "client error"
else puts "unknown"
end
```

### Loops

```
5.times { |i| puts i }
(1..10).each { |n| puts n }
while condition do end
until condition do end
loop { break if done }
```

### Ternary & Logical

```
status = age >= 18 ? "adult" : "minor"
name = input || "default"     # or-assign
name ||= "fallback"           # same effect
```

## Methods

### Defining Methods

```
def greet(name, greeting = "Hello")
  "#{greeting}, #{name}!"
end
greet("Alice")          # "Hello, Alice!"
greet("Bob", "Hi")     # "Hi, Bob!"
```

### Return Values

```
def add(a, b)
  a + b    # last expression is implicit return
end
def divide(a, b)
  return nil if b == 0
  a.to_f / b
end
```

### Keyword & Splat Arguments

```
def connect(host:, port: 80, **opts)
  puts "#{host}:#{port} #{opts}"
end
def log(*messages)
  messages.each { |m| puts m }
end
```

### Method Conventions

| Command | Description |
|---------|-------------|
| `method?` | Returns boolean (predicate) |
| `method!` | Mutates receiver (bang method) |
| `self.method` | Class method definition |

## Classes

### Class Definition

```
class User
  attr_accessor :name, :email
  def initialize(name, email)
    @name = name
    @email = email
  end
end
```

### Inheritance

```
class Admin < User
  def initialize(name, email, level)
    super(name, email)
    @level = level
  end
end
```

### Access Control

| Command | Description |
|---------|-------------|
| `public` | Default; accessible from anywhere |
| `private` | Only accessible within the class |
| `protected` | Accessible within class and subclasses |
| `attr_reader` | Generate getter method |
| `attr_writer` | Generate setter method |
| `attr_accessor` | Generate getter and setter |

## Modules

### Mixins

```
module Greetable
  def greet
    "Hello, I'm #{name}"
  end
end
class User; include Greetable; end
```

### Namespaces

```
module Payment
  class Processor
    def charge(amount) end
  end
end
p = Payment::Processor.new
```

### Include vs Extend

| Command | Description |
|---------|-------------|
| `include ModName` | Add as instance methods |
| `extend ModName` | Add as class methods |
| `prepend ModName` | Insert before class in method lookup |

## Blocks & Iterators

### Block Syntax

```
[1, 2, 3].each { |n| puts n }        # single-line block
[1, 2, 3].each do |n|
  puts n                               # multi-line block
end
```

### Yield

```
def with_logging
  puts "start"
  result = yield
  puts "end"
  result
end
with_logging { expensive_operation }
```

### Procs & Lambdas

```
square = Proc.new { |x| x ** 2 }
square.call(5)                   # 25
double = ->(x) { x * 2 }        # lambda
double.call(3)                   # 6
[1, 2, 3].map(&square)           # [1, 4, 9]
```

### Common Iterators

| Command | Description |
|---------|-------------|
| `.each` | Iterate over elements |
| `.map / .collect` | Transform each element |
| `.select / .filter` | Keep matching elements |
| `.reject` | Remove matching elements |
| `.reduce / .inject` | Accumulate into single value |
| `.each_with_index` | Iterate with index |
| `.flat_map` | Map and flatten one level |
| `.any? / .all? / .none?` | Boolean checks on collection |

## Regex

### Matching

```
"hello 42" =~ /\d+/         # 6 (match position)
"hello" =~ /\d+/            # nil (no match)
"hello".match?(/ell/)       # true
md = "age: 30".match(/(\d+)/)
md[1]                        # "30"
```

### Common Patterns

| Command | Description |
|---------|-------------|
| `/^start/` | Anchored at beginning |
| `/end$/` | Anchored at end |
| `/\d+/` | One or more digits |
| `/\w+/` | Word characters |
| `/\s+/` | Whitespace |
| `/[a-z]+/i` | Case-insensitive |
| `/(group)/` | Capture group |

### Substitution

```
"hello world".sub(/world/, "Ruby")     # first match
"aabba".gsub(/a/, "x")                 # all matches: "xxbbx"
"foo bar".gsub(/(\w+)/) { $1.upcase }  # "FOO BAR"
```

## File I/O

### Read & Write

```
content = File.read("data.txt")
lines = File.readlines("data.txt", chomp: true)
File.write("out.txt", "hello\n")
File.open("log.txt", "a") { |f| f.puts "entry" }
```

### File Operations

| Command | Description |
|---------|-------------|
| `File.exist?(path)` | Check if file exists |
| `File.directory?(path)` | Check if path is a directory |
| `File.basename(path)` | Filename without directory |
| `File.extname(path)` | File extension |
| `File.size(path)` | File size in bytes |
| `File.delete(path)` | Delete a file |
| `Dir.glob('*.rb')` | Find files matching pattern |
| `FileUtils.mkdir_p(path)` | Create directory recursively |

### CSV & JSON

```
require "json"
data = JSON.parse(File.read("data.json"))
File.write("out.json", JSON.pretty_generate(data))
require "csv"
CSV.foreach("data.csv", headers: true) { |row| puts row["name"] }
```